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¹⁰He has not been observed. It has been searched for in the spontaneous fission of ²⁵²Cf (82AL1C), in the fragmentation of 0.79 GeV/A ¹¹Li ions (87KO1Y) and in the fragmentation of a 30 MeV/A ¹⁸O beam (88ST06). The production rate in the latter experiment is < 3 x 10^-5 of the measured production probability of ⁸He (88ST06). See also (84AJ01). The calculated value of the atomic mass excess of ¹⁰He is 48.92 ± 0.14 MeV: ¹⁰He is then unstable with respect to breakup into ⁹He + n and ⁸He + 2n by 0.04 and 1.18 MeV, respectively (88BRZZ). See also (84BE1C), (79AJ01, 84AJ01, 87FL1A, 87HA1R, 87PE1C, 87SE1B) and (83ANZQ, 83PO1A, 84VA06, 85PO10, 85SA32, 86SA30, 87BL18, 87SA15; theor.).